Brain abnormalities associated with substance use disorders (SUDs) involve long-term changes in gene expression that derail motivational circuits toward drug seeking and taking despite negative consequences. While the role of transcription factors in gene expression changes has been well established, the subsequent post-transcriptional events that result in functional change are far from understood. Recent breakthroughs suggest that circular RNAs (circRNAs) are critical in regulating post-transcriptional events. This new class of non-coding RNAs plays a critical role in brain development and synaptic function by acting as sponges for sequestering microRNAs (miRNAs) and RNA-binding proteins (RBPs), which regulate gene expression, often in a competitive manner. For instance, our laboratory has shown that the RBP, HuD, competes with miR-495 to regulate expression of addiction-related genes. Our new preliminary results demonstrate that HuD also binds to, and regulates, the expression and synaptic localization of 14 brain-specific circRNAs, including circHomer1. circHomer1 is generated from the same gene (Homer1) that generates linear Homer1b mRNA, which is also a HuD target. Importantly, Homer1 protein is involved in cocaine?induced plasticity and drug seeking via its role in regulating type I metabotropic glutamate receptor signaling and homeostatic synaptic downscaling after increased neuronal activity. We found that knockdown of circHomer1 levels increases synaptic activity and synaptic Homer1b mRNA levels. Using HuD overexpressing mice, we determined that this process requires binding of the circRNA to HuD. Furthermore, we found that circHomer1 levels in the nucleus accumbens (NAc) are decreased in mice displaying increased cocaine-seeking behavior in the conditioned place preference model. In contrast, the varying levels of operant cocaine-seeking behavior in rats living in an enriched environment (low levels) versus isolation (high levels) during forced abstinence after a history of cocaine self- administration inversely correlate with the ratios of circHomer1 to Homer1b mRNA levels in the NAc shell. Based on these results, we hypothesize that synaptic circHomer1 expression in the NAc shell regulates synaptic activity by competing with Homer1b mRNA for HuD binding, leading to downstream effects in Homer1b function that attenuate drug-seeking behavior. To test this hypothesis, we will first examine whether circHomer1 and Homer1b mRNA compete for HuD binding, transport to synapses, and the control of neuronal activity in neurons in culture. We will then evaluate the effects of lentiviral-mediated manipulations of circHomer1 levels on Homer1b mRNA synaptic levels and the modulation of operant cocaine-seeking behavior. This research will impact the field by elucidating the role of circHomer1 in the mechanisms underlying changes in cocaine-induced neuroplasticity and addiction-related behaviors. Ultimately, gaining an understanding of the molecular mechanisms underlying brain changes in SUDs will aid in developing novel treatment strategies for cocaine use disorders.